1. Field of the Invention
The present invention is related to digital radar systems and, more particularly, to providing a low, constant false alarm rate in such a radar system.
2. Description of the Related Art
Radar systems used for air traffic control, as well as those used for other purposes, receive echo signals from many sources, including ground clutter, such as trees, buildings, mountains, etc., birds and rainstorms, for example, as well as aircraft. Many different types of processing are applied to received radar signals to identify aircraft, which are termed targets, and ignore other echo signals. Different techniques are used to eliminate different types of unwanted echo signals. As an example of a technique for eliminating echo signals from birds, see U.S. patent application Ser. No. 07/267,091, incorporated herein by reference. One of the techniques that is used to eliminate many types of unwanted echo signals is Doppler filtering. A received radar signal may be separated into several different Doppler bands representing the strength of return at various velocities toward or away from the radiating source of the radar signals.
Typically, after Doppler filtering, the received signals in each of the Doppler bands is processed by a constant false alarm rate (CFAR) circuit to remove echo signals which are unlikely to be a target. The received signals are separated into range cells, each cell corresponding to a different distance from the radiating source. The range cells are edited to remove saturated echo signals, i.e., the power of the received signal in the range cell is above the limit of the receiver. In addition, range cells containing echo signals from another pulsed radiating source are excluded from further consideration. These range cells are identifiable due to the use of a different pulse repetition sequence by other radiating sources.
The remaining range cells are supplied to a conventional CFAR circuit. As illustrated on line A of FIG. 1, a group of cells (L), e.g., cells 14-25, received before a cell of interest (I), e.g., cell 27 form a lag window. The lag window is typically separated from the cell of interest I by at least one cell, termed a guard cell (G), e.g., cell 26. Similarly, a group of cells (D), e.g., cells 29-40, received after the cell of interest I form a lead window and are separated from the cell of interest I by another guard cell (G), e.g., cell 28.
In the CFAR circuit each of the lead and lag windows typically undergo processing to eliminate three range cells, the range cell with the highest power level and two range cells in the window which are closest thereto to enable detection of two aircraft spaced closely in range, i.e., within one window. For example, if range cell 20 has the highest power level of any of the range cells 14-25, range cells 19-21 will be edited out and the power levels of the remaining range cells 14-18 and 22-25 will be summed to produce a power sum. Similarly, if cell 25 has the highest power level of cells 14-25, cells 14-22 will be summed to produce a power sum. A mean level adjustment value is added to the power sum to produce a mean level threshold for the lag window. A mean level threshold is produced in a similar manner for the lead window containing cells 29-40.
In a conventional CFAR circuit, the greater of the mean level thresholds calculated for the lead and lag windows is compared with the power level of the cell of interest I. If the power level of the cell of interest I is greater than the mean level threshold, a target is identified as existing in the cell of interest I. Subsequently, cell 28 which had been a guard cell G, will become the cell of interest I, the windows will shift one cell to the right and the process will be repeated for the new cell of interest I.
The two-window range-averaging CFAR ordinarily provides a satisfactory false alarm rate. However, turbulent areas at the edges of severe storms and severe weather cells in the center of larger storm systems, as well as near saturating ground clutter, can cause undesirable false alarms. An example of echo signals from a localized turbulent area in a severe storm is illustrated on line B of FIG. 1. When the cell of interest I is at the center of the turbulent area, the cell of interest I and guard cells G, i.e., cells 26-28, are automatically excluded and the editing of high power level cells will exclude cells 25 and 29 in the lag and lead windows, respectively, and the two cells closest thereto, i.e., cells 23 and 24 in the lag window and cells 30 and 31 in the lead window. As a result, the mean level thresholds of the lead and lag windows will use only the range cells in cells 32-40 and 14-22, respectively, which do not contain saturation or interference, as described above. As a result, a false target likely will be generated for the cell of interest I.